During August-September 2005, the New Hampshire Department of Health and Human Services reported seven cases of human eastern equine encephalitis virus (EEEV) disease, the first laboratory-confirmed, locally acquired cases of human EEEV disease reported from New Hampshire in 41 years of national surveillance. Also during August-September 2005, the Massachusetts Department of Public Health reported four cases of human EEEV disease, five times the annual average of 0.8 cases reported from Massachusetts during the preceding 10 years. Four of the 11 patients from New Hampshire and Massachusetts died. EEEV is transmitted in marshes and swamps in an enzootic bird-mosquito-bird cycle primarily by the mosquito Culiseta melanura. Bridge mosquito vectors (e.g., Coquillettidia perturbans, Aedes vexans, or Aedes sollicitans) transmit EEEV to humans and other mammals.1,2 This report summarizes the investigations of cases in New Hampshire and Massachusetts conducted by the two state health departments and CDC. The findings underscore the importance of surveillance for, and diagnostic consideration of, arboviral encephalitis in the United States and promotion of preventive measures such as local mosquito control and use of insect repellent.

A case of EEEV disease was defined as meningitis or encephalitis that occurred during July 1–September 30, 2005, in a resident of New Hampshire or Massachusetts with (1) anti-EEEV IgM antibody in cerebrospinal fluid (CSF) or (2) elevated anti-EEEV IgM antibody by IgM antibody capture enzyme-linked immunosorbent assay (MAC-ELISA) and neutralizing antibodies to EEEV by plaque-reduction neutralization test (PRNT) in serum. Interviews were conducted with patients, family members, or friends; medical records were reviewed; and homes and other potential mosquito-exposure sites were mapped and evaluated for the presence of mosquito-breeding sites.

Symptom onset occurred from the week beginning August 1 through the week beginning September 12. Median age of the patients was 45 years (range: 3 months to 85 years); six (55%) were male. All 11 patients were hospitalized; four (36%) died. Before hospitalization, three patients (27%) had symptoms lasting <1 day, and eight patients (73%) had symptoms lasting 2-15 days. Five patients, including the four who died, visited health-care providers for evaluation of nonspecific symptoms before being hospitalized with encephalitis or meningitis. Nine patients (82%) had encephalitis marked by altered mental status; of these, three had acute neurologic symptoms that required hospitalization on the same day, and the other six had neurologic symptoms after a prodrome of nonspecific systemic symptoms. Two (29%) had meningitis without altered mental status. Of 10 patients who had CSF samples collected, all had pleocytosis (range: 77-1,468 leukocytes/μL). EEEV was isolated from the cerebral cortex of one deceased patient who underwent autopsy. Serum samples from 26 family members or friends of patients in New Hampshire were tested for anti-EEEV IgM; none had IgM present in serum collected within 2 months of patient symptom onset.

Seven patients resided in three counties (Hillsborough, Merrimack, and Rockingham) in southeastern New Hampshire, and four resided in one county (Plymouth) in southeastern Massachusetts. All the patients worked or socialized in areas near swamps, cranberry bogs, or other wetlands capable of supporting production of bridge mosquito populations and both epizootic and enzootic transmission. In addition, all patients lived in wooded areas within a half mile of a swamp or cranberry bog and had potential outdoor exposure at dawn or dusk during the 2 weeks preceding illness onset. Information regarding insect repellent use was collected from six patients by direct or parental interview; one reported always using repellent, two reported occasional repellent use, and three reported never using repellent.

In New Hampshire and Massachusetts, mosquito pools (i.e., collections of 50 mosquitoes sorted by species and sex) were homogenized and tested for the presence of EEEV by reverse transcription–polymerase chain reaction (RT-PCR). The New Hampshire Department of Health and Human Services tested 3,938 mosquito pools and determined that 15 (0.4%) pools from four counties were EEEV positive: 10 Culiseta morsitans, two Culiseta melanura, one Coquillettidia perturbans, one Culex pipiens, and one Aedes cinereus. The Massachusetts Department of Public Health tested 8,136 mosquito pools and determined that 45 (0.6%) pools from six counties were EEEV positive: 41 Culiseta melanura, two Coquillettidia perturbans, one Culex pipiens-restuans, and one Ochlerotatus japonicus japonicus.

Specimens from animals suspected of having EEEV disease were submitted to the two state health departments and, if accepted, tested by RT-PCR, MAC-ELISA, or PRNT. In New Hampshire, 241 wild birds were tested, and 52 were EEEV positive; 33 veterinary animals were tested, and 16 animals (nine horses, four alpacas, two emus, and one llama) in seven counties were EEEV positive. In Massachusetts, wild birds were not tested; of 13 veterinary animals tested, five animals (four horses and one emu) in four counties were EEEV positive.

EEEV causes sporadic human disease in areas where the virus is endemic. Of the four lineages of EEEV, Group I is endemic in North America and the Caribbean and causes the majority of human disease; the other three groups (IIA, IIB, and III) cause primarily equine illness in Central and South America. For 2005, a total of 21 confirmed or probable cases of human EEEV disease* were reported to CDC, compared with 41 during 2000-2004,† an average of 8.2 cases per year. States reporting the highest annual average number of cases of EEEV disease during 2000-2004 were Florida (1.4 cases) and Michigan (1.2), followed by Georgia, Massachusetts, North Carolina, and South Carolina (0.8 each). Although few cases have been reported, EEEV disease can have severe health and economic consequences. The fatality rate has been estimated at 35%-75%,1-4 and eastern equine encephalitis can result in long-term neurologic sequelae, which, in one study, were projected to result in lifetime disease-related expenses of $3 million per patient.5

EEEV disease occurs near habitats suitable for breeding enzootic and bridge vectors and where avian amplifying hosts are abundant. A serosurvey of residents in towns with cases of EEEV disease during a 1959 New Jersey outbreak revealed an EEEV antibody seroprevalence of 2%-6% and a ratio of apparent to inapparent infections ranging from 1:16 to 1:32 (mean: 1:23).6 Measures to control EEEV disease and other mosquito-borne diseases have focused on mosquito-control programs and public education regarding personal protection against mosquito bites. Massachusetts has local mosquito-control districts that routinely collect and submit mosquito pools to the state public health laboratory for testing. New Hampshire has no statewide testing program, but 16 towns and cities in 2005 funded their own mosquito surveillance and sent mosquito pools to the state for testing. In response to the 2005 outbreak, New Hampshire (1) began a public education campaign; (2) heightened human, equine, and avian surveillance for EEEV disease; and (3) trapped mosquitoes around patient residences and other potential exposure sites. In addition, the New Hampshire House of Representatives passed a bill that establishes a mosquito-control fund to assist towns, cities, and mosquito-control districts and a task force to facilitate a coordinated local, regional, and state response to arboviral disease.‡ Massachusetts is continuing its ongoing mosquito surveillance and public education campaigns.

Patients with aseptic meningitis or encephalitis in areas that support EEEV transmission should be tested for EEEV disease, and health-care providers should alert their state health departments when human or veterinary EEEV disease is suspected. Public health practitioners should advise the public to avoid EEEV disease and other mosquito-borne diseases by using personal protective measures (e.g., regular use of insect repellents containing DEET, picaridin, or oil of lemon eucalyptus7; wearing long-sleeved shirts and pants when outdoors; and avoiding outdoor exposure during periods when mosquitoes are most actively biting, usually from dusk to dawn). Communities in which risk for transmission of EEEV has been demonstrated should consider establishing mosquito surveillance and control programs.